Estuaries are transitional ecosystems where freshwater and marine waters meet, and their biodiversity overlaps. As a result of their supporting roles in trade, transport, fishing and tourism, estuaries are often also highly altered and pressurised ecosystems.

According to a new study, European estuaries are home to some of the most vulnerable and least resilient estuarine fish populations in the world. Writing in Nature Scientific Reports, Rita P. Vasconcelos and colleagues outline that European estuaries are particularly pressurised by human activities such as overfishing, habitat alteration and pollution. However, the authors of the newly published study argue that these highly-pressurised European estuaries are often lacking in sufficient protected area coverage to help conserve vulnerable fish species.

The researchers used publicly-available scientific data sets on global fish populations for their analysis. They found that two fish species traits which affect species response to pressures – vulnerability and resilience – were not evenly distributed in estuarine populations globally, and were driven by environmental features such as temperature, connectivity and area.

In this study, both species traits are based on responses to fishing pressures. Vulnerability describes the susceptibility of fish species to become extinct due to fishing pressure. Vulnerability is an indirect measure of species sensitivity to change, and can be influenced by life history and ecological features such as body size, age at maturity and geographical range.

Resilience is used in this study to describe the capacity of fish species to recover from fishing pressure. The resilience of fish species is related to their fecundity and natural rate of population growth. In essence, the use of these species traits in this study allows the authors to analyse the susceptibility of global estuarine fish biodiversity to multiple human pressures.

The researchers outline that assemblages of more vulnerable and less resilient fish species were generally found in estuaries at higher latitudes – particularly Europe – and with high connectivity between the freshwater and marine environments. The authors argue that current conservation schemes typically pay little attention to species traits such as vulnerability and resilience, despite their ecological importance.

In this study, Vasconcelos and colleagues show that global protected area coverage of estuary ecosystems is only weakly related to the distribution of sensitive fish species. This shortfall in protection is exacerbated by the multiple pressures acting on the estuaries supporting the most vulnerable fish populations. They argue that conservation schemes are needed in estuary environments across the world, particularly those with populations of vulnerable fish species.

Their study is focused on protected area management, where restrictions are placed on human activities within a set geographical area, such as those classified in IUCN management categories I-V. In addition, it is largely focused on fishing as a key pressure on fish populations, as measured through their trait responses. As such, the study doesn’t address the environmental benefits that legislation such as the Water Framework Directive brings to European estuaries, by requiring governments to conserve and restore water and habitat quality.

Regardless, the study has an important message: European estuaries subject to multiple pressures are home to vulnerable fish populations, which are in need of protection.

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